Liquid crystal display device and method for manufacturing liquid crystal display device

ABSTRACT

A liquid crystal display device includes a pair of substrates of which one substrate is provided with a plurality of scanning lines and a plurality of common wirings, a first insulation film covering the scanning lines, the common wirings, and the one substrate, a plurality of signal lines provided on the first insulation film, a thin film transistor provided near an intersection part of the scanning lines and the signal lines, a lower electrode formed below the first insulation film and connected to the common wirings, a second insulation film formed on surfaces of the thin film transistor, the signal lines, and the first insulation film, and an upper electrode formed on the second insulation film and having a slit, a display region in which the liquid crystal layer is driven by an electric field, and a non-display region that is formed outside the display region.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application contains subject matter relating JapanesePriority Patent Application JP 2009-075839 filed in the Japan PatentOffice on Mar. 26, 2009, the entire contents of which are herebyincorporated by reference.

BACKGROUND

The present application relates to a liquid crystal display device of afringe field switching (referred to below as FFS) mode. Especially, thepresent technology relates to a liquid crystal display device of the FFSmode in which short-circuiting between a signal line and a commonelectrode (also referred to below as a lower electrode) is suppressed.

Examples of liquid crystal display devices employing a lateral electricfield system include a liquid crystal display device of the FFS mode inwhich a pair of electrodes which are a pixel electrode and a commonelectrode is provided only on one substrate. In the liquid crystaldisplay device of the FFS mode, the pixel electrode and the commonelectrode which are used for applying an electric field to a liquidcrystal layer are respectively disposed on different layers with aninsulation film interposed. The liquid crystal display device of the FFSmode has a wide visual angle, high contrast capability, and highertransmittance and can be driven by low voltage, being able to performbright display. In addition, the liquid crystal display device of theFFS mode has a large overlapping area of the pixel electrode and thecommon electrode when viewed from above so as to have such advantagethat larger storage capacitance is additionally produced and thereforeprovision of a separate auxiliary capacitance electrode is not demanded.

However, in manufacturing of a liquid crystal display device, physicalvapor deposition such as vacuum vapor deposition and sputtering ororganometallic chemical vapor deposition by thermal decomposition hasbeen employed as a deposition method in the related art. Therefore, insuch liquid crystal display device of the FFS mode, a step is formed ona position on which a signal line and a common wiring intersect witheach other due to lamination of the signal line and the common wiring,and the film thickness of a lateral surface region of the step decreasesso as to more likely cause decrease of dielectric pressure. Accordingly,failures such as disconnecting and short-circuiting have sometimesappeared. In the liquid crystal display device of the FFS mode of therelated art, a surface of the common wiring is covered by a lowerelectrode, a surface of the lower electrode is covered by a gateinsulation film, a signal line is formed on a surface of the gateinsulation film, and a thin film transistor TFT serving as a switchingelement is formed near an intersection part of a scanning line and thesignal line. Therefore, in the manufacturing, after an amorphous silicon(a-Si) layer and an n+a-Si layer, for example, are formed on the wholesurface of the gate insulation film, patterning is performed to form asemiconductor film for forming a TFT by photolithography. At this time,before the a-Si layer and the n+a-Si layer are patterned by thephotolithography, there is a cleaning process by using pure water. Inthe cleaning, static electricity is generated between the pure water andthe n+a-Si layer, and spark is generated between the n+a-Si layer andthe common wiring due to the static electricity, so that dielectricbreakdown may occur in a first insulation film which is interposedbetween n+a-Si layer and the lower electrode.

In formation of a metallic film on a formed fine step in a semiconductorsubstrate, a ratio between the film thickness in a lateral surfaceregion of the step and the film thickness in a flat part around the stepis called step coverage. When the film thickness in the flat part aroundthe step is denoted as A and the film thickness in the lateral surfaceregion of the step is denoted as B, the step coverage is expressed asB/A. As this value becomes larger than 1, the coverage property becomesbetter, while as this value becomes smaller than 1, the coverageproperty becomes poorer. When the film thickness of the lateral surfaceregion of the step is smaller than the film thickness in the flat partaround the step, the coverage property is poor, easily causing fineholes or cracks in the lateral surface region of the step.

If a signal line, a source electrode, a drain electrode, and the likeare patterned after a source layer is formed on a surface of the firstinsulation film in this state, the source layer enters a broken part ofa gate insulation film. Accordingly, the signal line and the lowerelectrode short-circuit, sometimes exhibiting a line defect. Suchphenomenon occurs because the thickness of the gate insulation filmcovering lateral surfaces of the common wiring and the lower electrodeis small. The small thickness is caused by the large step formed by thecommon wiring and the lower electrode and the poor step coverage of thegate insulation film covering the common wiring and the lower electrode.Causes that the step formed by the common wiring and the lower electrodebecomes large are the following: the lateral surface is formed on thesame position of the lateral surface of the common wiring so as toprevent the lower electrode from protruding to the scanning line side(1) because a common wiring is shifted and disposed to a side of one ofthe scanning lines adjacent to the region in which this common wiring isprovided, so as to improve aperture ratio and display image quality, and(2) because a contact area between the common wiring and the lowerelectrode should be enlarged as much as possible so as to reduce contactresistance between the common wiring and the lower electrode.

For such problem, Japanese Unexamined Patent Application Publication No.6-112333 discloses a semiconductor device that can secure sufficientdielectric breakdown strength against static electricity by doubling aninsulation film. In the semiconductor device disclosed in JapaneseUnexamined Patent Application Publication No. 6-112333, the insulationfilm has the double-layer configuration which is composed of a firstinsulation film which is a silicon oxide film or a silicon nitride filmand a second insulation film made of a heat-resistant organic material.Further, the second insulation film is formed by depositing a liquidsubstance having a large viscosity, so that a corner of the secondinsulation film does not have a cliff shape but has a loose shape andthus the film edge part becomes loose to have a gentle slope shape(taper shape). Therefore, the film thickness of the insulation film canbe increased compared to the related art, being able to securesufficient dielectric breakdown strength against static electricity.

SUMMARY

However, in the method that sufficient film thickness is obtained byforming the insulation film to have the edge part having the slope shapeas the semiconductor device disclosed in Japanese Unexamined PatentApplication Publication No. 6-112333, even though the edge part of theinsulation film is formed to have the slope shape and the step coverageis improved, the film thickness of the insulation film formed on theupper part side of the edge part of the lower electrode remains smallerthan the flat part. Thus, it is difficult to secure sufficient filmthickness over the whole insulation film. Therefore, in thesemiconductor device disclosed in Japanese Unexamined Patent ApplicationPublication No. 6-112333 as well, electrostatic breakdown may locallyoccur on a part having the smaller thickness and formed on the upperside of the slope-shaped edge part of the lower electrode due to anelectric field generated between the static electricity generated in thecleaning and the lower electrode.

After a great deal of consideration to deal with the above-mentionedproblems of the related art, the inventors found that insufficientformation of the film thickness of an edge part could be dissolved informing a CVD of a first insulation film by extending a lower electrodewhich is formed on a common wiring which is an origin of an occurrenceof spark and thus forming a plurality of steps. It is desirable toprovide a highly reliable liquid crystal display device of the FFS modein which short-circuiting between a lower electrode and a signal line issuppressed and an occurrence of a line defect is reduced.

Further, it is desirable to provide a method for manufacturing a liquidcrystal display device of the FFS mode which exhibits theabove-described advantageous effects.

According to an embodiment, there is provided a liquid crystal displaydevice including a pair of substrates that sandwich and hold a liquidcrystal layer and of which one substrate is provided with a plurality ofscanning lines and a plurality of common wirings that are providedparallel with each other, a first insulation film that covers thescanning lines, the common wirings, and the one substrate, a pluralityof signal lines that are provided on the first insulation film in adirection intersecting with the scanning lines and the common wirings, athin film transistor that is provided near an intersection part of thescanning lines and the signal lines, a lower electrode that is formedbelow the first insulation film and is connected to the common wirings,a second insulation film that is formed on surfaces of the thin filmtransistor, the signal lines, and the first insulation film, and anupper electrode that is formed on the second insulation film to overlapwith the lower electrode when viewed from above and has a slit, adisplay region in which the liquid crystal layer is driven by anelectric field generated between the lower electrode and the upperelectrode, and a non-display region that is formed outside the displayregion. In the liquid crystal display device, each of the common wiringsis provided in a manner to be shifted on a side of one of the scanninglines that are adjacent to a region in which the common wiring isprovided, and the lower electrode covers the common wirings in a mannerthat a plurality of steps are formed on at least one lateral surface ofthe common wiring.

The liquid crystal display device of the embodiment includes a pair ofsubstrates that sandwich and hold a liquid crystal layer and of whichone substrate is provided with a plurality of scanning lines and aplurality of common wirings that are provided parallel with each other,a first insulation film that covers the scanning lines, the commonwirings, and the one substrate which is exposed, a plurality of signallines that are provided on the first insulation film in a directionintersecting with the scanning lines and the common wirings, a thin filmtransistor (TFT) that is provided near an intersection part of thescanning lines and the signal lines, a lower electrode that is made of atransparent conductive material, is formed below the first insulationfilm in every region partitioned by the plurality of scanning lines andthe signal lines, and is connected to the common wirings, a secondinsulation film that is formed on surfaces of the thin film transistorand its electrode, the signal lines, and the first insulation film whichis exposed, and an upper electrode that is made of a transparentconductive material, is formed on the second insulation film to overlapwith the lower electrode when viewed from above, and has a plurality ofslits which are provided to be parallel with each other. Accordingly,the liquid crystal display device of the embodiment operates as a liquidcrystal display device of the FFS mode. Here, the lower electrode servesas a common electrode and the upper electrode serves as a pixelelectrode.

In the liquid crystal display device of the embodiment, the lowerelectrode is formed to cover the common wiring in a manner that a stepis formed on the lateral surface of the common wiring. Thus, a pluralityof steps is formed on the common wirings and the lower electrode. Withsuch configuration, even though surfaces of these layers are covered bythe first insulation film, the thickness reduction of the firstinsulation film is suppressed because individual steps are small.Accordingly, above-described dielectric breakdown of the firstinsulation film caused by static electricity in a manufacturing processsuch as cleaning is suppressed. According to the liquid crystal displaydevice of the embodiment, short-circuiting between the signal line andthe common wiring is suppressed, so that a liquid crystal display devicein which an occurrence of a line defect is reduced can be provided.

Further, in the liquid crystal display device of the embodiment, thelower electrode covers the surface of the common wiring disposed betweenlower electrodes. It is commonly sufficient that the lower electrode isformed in every region partitioned by the plurality of scanning linesand the signal lines. If the lower electrode covers also the surface ofthe common wiring disposed between the lower electrodes, no step isgenerated on the first insulation film on both sides of the signal lineson a part on which the signal lines and the common wirings intersectwith each other. Therefore, dielectric breakdown of the first insulationfilm caused by static electricity generated in the cleaning can befurther suppressed.

Further, in the liquid crystal display device of the embodiment, thecommon wiring is provided in a manner to be shifted to a side of one ofthe scanning lines that are adjacent to the pixel region in which thiscommon wiring is provided. With such configuration, the common wiring isnot positioned on the center of one pixel, increasing an aperture ratio.Further, one pixel is not divided into two regions by the common wiring,improving display image quality.

In the liquid crystal display device of the embodiment, it is preferablethat a width of a step part of the lower electrode be 2.25 μm or morefrom the lateral surface of the common wirings.

The thickness of the scanning line and the thickness of the commonwiring are approximately 0.2 μm, the thickness of the lower electrode isapproximately 0.1 μm, and the thickness of the first insulation filmwhich is also called a gate insulation film is commonly approximately0.4 μm. Therefore, by setting the width of the step part of the lowerelectrode to be 2.25 μm or more from the lateral surface of the commonwiring, even if variety of the line width of the signal lines or patternmisalignment is taken into consideration, the first insulation filmwhich is formed on the lower electrode can be set to have the thicknessby which dielectric breakdown caused by static electricity can besufficiently suppressed on the lateral surface of the step part as well.According to the liquid crystal display device of the embodiment, aliquid crystal display device in which short-circuiting between thesignal lines and the common wirings can be further suppressed and anoccurrence of a line defect is reduced can be provided. If the width ofthe step of the lower electrode is less than 2.25 μm, short-circuitingbetween the lower electrode and the signal lines more frequently occursdisadvantageously.

In the liquid crystal display device of the embodiment, it is preferablethat the step part of the lower electrode be formed only near a positionon which the common wirings and the signal lines intersect with eachother.

The embodiment is intended to prevent short-circuiting between the lowerelectrode and the signal lines, so that it is sufficient that the steppart of the lower electrode is formed only on a part on which the commonwirings and the signal lines intersect with each other. According to theliquid crystal display device of the embodiment, a liquid crystaldisplay device in which an occurrence of a line defect is suppressed,the aperture ratio is large, and display image quality is superior canbe provided.

In the liquid crystal display device of the embodiment, it is preferablethat the first insulation film be formed such that when a thickness ofan insulation film formed on a flat part on the lower electrode isdenoted as A and a thickness of an insulation film formed on a lateralsurface of the steps of the lower electrode is denoted as B, a stepcoverage of the first insulation film satisfy a relationship of B/A≧1.

If the step coverage of the first insulation film is set to satisfy therelationship of B/A≧1, it can be determined that the film thickness ofthe lateral surface region of the step is sufficiently large compared tothe film thickness in the flat part around the step and the coverageproperty is superior, being able to impart sufficient dielectricpressure to the first insulation film. Therefore, according to theliquid crystal display device of the embodiment, the dielectric strengthof the first insulation film can be increased, so that a liquid crystaldisplay device in which short-circuiting between the signal lines andthe common wirings can be further suppressed and an occurrence of a linedefect is reduced can be provided. If the step coverage is expressed asB/A≦1, the film thickness of the lateral surface region of the step issmall compared to the film thickness of the flat part around the step.Therefore, fine holes or cracks are easily generated in the lateralsurface region of the step and short-circuiting between the lowerelectrode and the signal lines more frequently occurs disadvantageously.

In the liquid crystal display device of the embodiment, it is preferablethat a dummy pixel be formed in the non-display region, and the steppart of the lower electrode be formed in the dummy pixel.

The dummy pixel region is formed in a region adjacent to the displayregion, that is, the dummy pixel region is a region which does notcontribute to actual display. However, thanks to the provision of thedummy pixel, the film thickness of each layer in the display region andthe film thickness of each layer in the dummy pixels can be set to besame as each other. Therefore, adverse affect caused by adjacency withthe non-display region is less imparted to display image quality ofpixels in a peripheral part of the display region. Further, the dummypixel is formed in the periphery of the display region, so that thedummy pixel can absorb stress from outside such as static electricityand can suppress an occurrence of defects in pixels within the displayregion. According to the liquid crystal display device of theembodiment, short-circuiting between the signal lines and the commonwirings can be suppressed in the dummy pixel as well, so that a highlyreliable liquid crystal display device in which defects of displaypixels in the display region are suppressed by appropriate performanceof the dummy pixel can be provided.

A method for manufacturing a liquid crystal display device according toanother embodiment includes (1) covering a whole surface of atransparent substrate by a conductive layer and etching the conductivelayer so as to pattern a plurality of scanning lines having a gateelectrode part and a plurality of common wirings in parallel with eachother, (2) covering a whole surface of a substrate obtained in theprocess (1) by a transparent conductive layer and patterning lowerelectrodes so that the lower electrodes cover surfaces of the commonwirings disposed between the lower electrodes and extend over lateralsurfaces of the common wirings, on positions corresponding to respectivepixels, (3) covering a whole surface of a substrate obtained in theprocess (2) by a first insulation film, (4) covering a whole surface ofthe first insulation film by a semiconductor layer and etching thesemiconductor layer so as to pattern the semiconductor layer on aposition corresponding to a gate electrode part, (5) covering a wholesurface of a substrate obtained in the process (4) by a conductive layerand etching the conductive layer so as to pattern a signal line in adirection intersecting with the scanning lines and the common wiringsand pattern a drain electrode and a source electrode that iselectrically connected to the signal line in each of the pixels, (6)covering a whole surface of a substrate obtained in the process (5) by asecond insulation film, (7) forming a contact hole on the secondinsulation film which is positioned on the drain electrode of each ofthe pixels, (8) covering a whole surface of a substrate obtained in theprocess (7) by a transparent conductive layer and etching thetransparent conductive layer so as to pattern an upper electrode havinga plurality of slits in each of the pixels, and electrically conductingthe upper electrode and the drain electrode, and (9) disposing asubstrate obtained in the process (8) and a color filter substrateopposed to each other and filling a space between the substrates withliquid crystal.

According to the method for manufacturing a liquid crystal displaydevice according to the embodiment, a liquid crystal display device ofthe FFS mode which exhibits the above-described advantageous effects canbe manufactured.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plan view of a liquid crystal display device of a FFS modeaccording to an embodiment;

FIG. 2 is a plan view schematically showing a part, which corresponds totwo pixels, of an array substrate of the liquid crystal display deviceof the FFS mode according to the embodiment;

FIG. 3 is a schematic sectional view taken along a III-III line of FIG.2;

FIGS. 4A to 4F are sectional views of a part corresponding to theIII-III line of FIG. 2 and sequentially showing a manufacturing processof the array substrate corresponding to one pixel of the embodiment;

FIGS. 5A to 5C are sectional views sequentially showing themanufacturing process, which follows the process of FIGS. 4A to 4F, ofthe array substrate corresponding to of one pixel of the embodiment;

FIG. 6A is an enlarged plan view transparently showing a VIA part ofFIG. 2 to a signal line, and FIG. 6B is a sectional view taken along aVIB-VIB line of FIG. 6A;

FIG. 7A is a sectional view of a related art example corresponding toFIG. 6B, and FIG. 7B is a sectional view showing a spark state; and

FIG. 8A is a plan view showing a short-circuiting state of the relatedart example and corresponding to FIG. 6A, and FIG. 8B is a sectionalview taken along a VIIIB-VIIIB line of FIG. 8A.

DETAILED DESCRIPTION

Embodiments of the present application will be described below in detailwith reference to the drawings.

In respective drawings used for the description in this specification,scales of respective layers and respective members shown are adequatelychanged in the extent to which the layers and members can be recognizedin the drawings, and thus the layers and the members are not necessarilyshown in proportion to actual dimensions.

A liquid crystal display device 10 of the FFS mode according to anembodiment is described with reference to FIGS. 1 to 6B. The liquidcrystal display device 10 according to the embodiment includes an arraysubstrate AR, a color filter substrate CF, and a sealing member 25 whichbonds the substrates AR and CF to each other as shown in FIG. 1. In theliquid crystal display device 10, liquid crystal (not shown) is injectedinto a region surrounded by the array substrate AR, the color filtersubstrate CF, and the sealing member 25 from a liquid crystal injectionport 27 and the liquid crystal injection port 27 is sealed by a sealingmember 28. In the liquid crystal display device 10, a region surroundedby the sealing member 25 constitutes a display region 26, and a regionwhich is provided on the periphery of the display region 26 and in whichan image is not recognized constitutes a non-display region 29 of theliquid crystal display device 10.

The array substrate AR is formed such that various kinds of wirings fordriving the liquid crystal and the like are formed on a surface of afirst transparent substrate 11 which is made of glass or the like andhas a rectangular shape. The array substrate AR has the longer length inthe longitudinal direction that the color filter substrate CF so as tohave an extending part 11 a which extends outward when the substrates ARand CF are bonded to each other. On the extending part 11 a, a driver Drwhich is composed of an IC chip, an LSI, or the like which outputs adriving signal is provided.

On the array substrate AR of the liquid crystal display device 10 of theFFS mode of the embodiment, a plurality of scanning lines 12 and aplurality of common wirings 13 are formed on the whole surface of thetransparent substrate 11 by photolithography or etching so as to beparallel with each other (refer to FIGS. 2 and 4A). Here, in the liquidcrystal display device 10 of the embodiment, the common wiring 13 isdisposed in a manner to be shifted to a side of one scanning line 12 soas to improve an aperture ratio and display image quality.

Subsequently, the whole surface of the transparent substrate 11 on whichthe scanning lines 12 and the common wirings 13 are formed is covered bya transparent conductive layer made of indium tin oxide (ITO), indiumzinc oxide (IZO), or the like and a lower electrode 14 is formed byphotolithography or the like in the same manner. At this time, the lowerelectrode 14 covers surfaces of the common wirings 13 disposed betweenrespective pixels, further, the lower electrode 14 covers the surfacesof the common wirings 13 so that a plurality of steps 14 a having thewidth X are formed as shown in FIGS. 4B, 6A, and 6B. Here, a part of thelower electrode 14 is extended to a position to cover the scanning line12 of an adjacent pixel, on a position on which the lower electrode 14is overlapped with a signal line 17 when viewed from above.

After the lower electrode 14 is formed in such way, a first insulationfilm 15 which is a silicon nitride layer is formed to cover the wholesurface of the substrate (refer to FIG. 4C). At this time, since thefirst insulation film 15 is formed on the plurality of steps 14 a formedon the lower electrode 14, a plurality of steps are formed on the firstinsulation film 15 as well.

Subsequently, after the whole surface of the insulation film 15 iscovered by an a-Si layer 16 a and n+a-Si layer 16 b, a semiconductorlayer 16 composed of the a-Si layer 16 a and the n+a-Si layer 16 b isformed in a TFT forming region by photolithography or the like in thesame manner (refer to FIGS. 4D to 4F). A region, which corresponds to aposition on which the semiconductor layer 16 is formed, of the scanningline 12 constitutes a gate electrode G of a TFT.

Then, the whole surface of the transparent substrate 11 on which thesemiconductor layer 16 is formed is covered by a conductive layer, andthe signal line 17 and a drain electrode D are formed also byphotolithography or the like (refer to FIG. 5A). Both of a sourceelectrode S part and a drain electrode D part of the signal line 17 arepartially overlapped with the surface of the semiconductor layer 16.

Here, a case of the embodiment and a case of a related art example arecompared and explained with reference to FIGS. 7A to 8B. A process shownin FIG. 4F includes a process of cleaning the substrate by pure water WTafter the semiconductor layer 16 b is formed. In the related artexample, the lower electrode 14 has an edge part E and the firstinsulation film 15 is formed on the lower electrode 14 as shown in FIG.7A, so that the thickness B′ of the first insulation film formed on theedge part E is smaller than the thickness A′ of a flat part of the firstinsulation film 15, degrading a step coverage (B′/A′) of the firstinsulation film 15 formed on the common wiring 13.

At this time, the semiconductor layer 16 is formed also on the surfaceof the first insulation film 15 which is formed on the common wiring 13as shown in FIG. 7B and the substrate is cleaned in this state.Therefore, during the cleaning by the pure water WT, static electricitygenerated by friction between the pure water WT and the n+a-Si layer 16b travels through the n+a-Si layer 16 b and the a-Si layer 16 a andcauses a spark 22 with the lower electrode 14 which is formed on thecommon wiring 13. Accordingly, the first insulation film 15 which isthinly formed on the lower electrode 14 is broken and thus a damage 23is disadvantageously formed (refer to FIG. 8B). Then, if the signal line17 is formed on the first insulation film 15 while leaving the damage 23in the first insulation film 15 in the process shown in FIG. 4F, asource layer enters the damage 23, which is formed when the firstinsulation film 15 is broken, in a source layer formation. Accordingly,short-circuiting between the signal line 17 and the lower electrode 14occurs to cause a line defect.

In the embodiment, the lower electrode 14 is formed to cover the commonwirings 13 and have a plurality of steps 14 a having the width X, asshown in FIGS. 6A and 6B. Then, the first insulation film 15 is formedto cover the steps 14 a. Thus, the first insulation film 15 is formedsuch that the thickness A of the flat part and the thickness B of thestep 14 a are approximately same as each other. Accordingly, the stepcoverage (B/A) of the first insulation film 15 in this step part isimproved, namely, the first insulation film 15 is formed thicker thanthat of the related art example. Therefore, dielectric strength againststatic electricity is improved, and breaking of the first insulationfilm 15 caused by static electricity generated during cleaning by thepure water WT is suppressed. As a result, short-circuiting 24 betweenthe lower electrode 14 and the signal line 17 can be suppressed.

In the embodiment, a part of the lower electrode 14 is extended to aposition on which the lower electrode 14 covers the scanning line 12 ofan adjacent pixel, on a position overlapping with the signal line 17when viewed from above. Therefore, the above-described advantageouseffect generated between the signal line 17 and the common wiring 13 canbe expected also between the signal line 17 and the scanning line 12.Accordingly, in the embodiment, the short-circuiting which may occurbetween the signal line 17 and the scanning line 12 can also besuppressed.

Subsequently, in order to complete the liquid crystal display device 10of the embodiment, after the whole surface of this substrate is coveredby a second insulation film 18 which is a silicon nitride layer, acontact hole 19 is formed on the second insulation film 18 on a positioncorresponding to the drain electrode D so as to expose a part of thedrain electrode D (refer to FIG. 5B). Further, a transparent conductivelayer made of ITO or the like is formed to cover the whole surface, andan upper electrode 21 including a plurality of slits 20, which areparallel to each other, is formed on a part, which is surrounded by thescanning line 12 and the signal line 17, of the second insulation film18 also by photolithography or the like so as to have a pattern shown inFIG. 2 (refer to FIG. 5C). The slits 20 are used for generating a fringefield effect. The upper electrode 21 is electrically connected with thedrain electrode D via the contact hole 19, so that the upper electrode21 functions as a pixel electrode.

Then, by forming a predetermined alignment film (not shown) is formedover the whole surface, the array substrate AR is completed. The arraysubstrate AR manufactured as described above and the color filtersubstrate which is separately manufactured are faced to each other, thenthe peripheries of the substrates are bonded to each other with thesealing member 25, and a space formed between the substrates is filledwith liquid crystal. Accordingly, the liquid crystal display device 10of the FFS mode according to the embodiment is obtained. The detaileddescription of the color filter substrate CF is omitted, but the colorfilter substrate CF has the substantially same configuration as that ofa liquid crystal display device of a twisted nematic (TN) mode of therelated art except that a color filter layer, an overcoat layer, and analignment film are layered on a surface of the transparent substratemade of glass or the like and no common electrode is provided.

According to the liquid crystal display device of the FFS mode of theembodiment which is manufactured as described above, the firstinsulation film having favorable step coverage is formed on the surfaceof the lower electrode, so that breaking of the first insulation filmdue to static electricity is suppressed. As a result, short-circuitingbetween the lower electrode and the signal line can be suppressed andaccordingly, a highly reliable liquid crystal display device can beprovided.

In the example of the embodiment, the steps of the lower electrode areformed on the whole surface along the common wiring. However, the stepsmay be formed only near a position on which short-circuiting easilyoccurs, for example, a position on which the common wiring and thesignal line intersect with each other. Accordingly, short-circuitingbetween the lower electrode and the signal line can be prevented andtherefore a liquid crystal display device of the FFS mode with superiordisplay quality can be provided.

It is favorable to set the width X of the plurality of steps of thelower electrode to be 2.25 μm or more so as to be able to form the firstinsulation film with sufficient thickness. The thickness of the lowerelectrode is approximately 0.1 μm, while the thickness of the firstinsulation film which is called a gate insulation film should beapproximately 0.4 μm commonly. By setting the width X of the pluralityof steps of the lower electrode to be 2.25 μm or more, even if varietyof the line width of the signal line or pattern misalignment is takeninto consideration, the first insulation film which is formed on thelower electrode can be set to have the thickness by which dielectricbreakdown due to static electricity can be sufficiently suppressed alsoon the lateral surface of the step part. If the width X of the pluralityof steps of the lower electrode is smaller than 2.25 μm,short-circuiting between the lower electrode and the signal line morefrequently occurs disadvantageously.

It is preferable that a measuring part of the film thickness A of theflat part of the first insulation film which is used for calculation ofa value of the step coverage (B/A) in the embodiment be the center ofthe flat part of the step uppermost part or the flat part of the stepperiphery, and it is preferable that a measuring part of the filmthickness B of the lateral surface region be the thinnest part of thestep part. Accordingly, a value of the step coverage (B/A) can beaccurately calculated.

It is favorable that the value of the step coverage (B/A) which is aratio of the film thickness A of the first insulation film and the filmthickness B of the lateral surface region in the embodiment is set to be1 or more. Accordingly, the film thickness B of the lateral surfaceregion is sufficiently thick compared to the film thickness A,substances for forming a film sufficiently remain also on the lateralsurface region of the step part, and therefore, a metal film or aninsulation film can be stably formed without generation of fine holes oran occurrence of cracks. Accordingly, sufficient dielectric pressure canbe imparted to the first insulation film, so that the dielectricstrength of the first insulation film can be increased andshort-circuiting between the signal line and the common wiring can bemore suppressed, being able to provide a liquid crystal display devicein which an occurrence of a line defect is reduced. If the value of thestep coverage (B/A) is less than 1, the film thickness B of the lateralsurface region is small compared to the film thickness A of the flatpart, substances for forming a film do not sufficiently remain on thelateral surface region of the step part as well, and fine holes orcracks are generated. Therefore, short-circuiting between the lowerelectrode and the signal line more frequently occurs disadvantageously.

Further, it is favorable that a dummy pixel is formed in the non-displayregion 29 of the liquid crystal display device of the embodiment and thestep part of the lower electrode is formed on the dummy pixel. The dummypixel is preferentially broken by static electricity and thus also has afunction to prevent the static electricity from adversely affecting thepixel within the display region. Therefore, the dummy pixel can preventthe static electricity from adversely affecting the pixel in the displayregion as long as the dummy pixel functions properly, being able toprovide a further highly reliable liquid crystal display device.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope and without diminishing itsintended advantages. It is therefore intended that such changes andmodifications be covered by the appended claims.

The application is claimed as follows:
 1. A liquid crystal displaydevice, comprising: a pair of substrates that sandwich and hold a liquidcrystal layer and of which one substrate is provided with, a pluralityof scanning lines and a plurality of common wirings, the scanning linesand the common wirings being provided parallel with each other, a firstinsulation film that covers the scanning lines, the common wirings, andthe one substrate, a plurality of signal lines that are provided on thefirst insulation film in a direction intersecting with the scanninglines and the common wirings, a thin film transistor that is providednear an intersection part of the scanning lines and the signal lines, alower electrode that is formed below the first insulation film and isconnected to the common wirings, a second insulation film that is formedon surfaces of the thin film transistor, the signal lines, and the firstinsulation film, and an upper electrode that is formed on the secondinsulation film to overlap with the lower electrode when viewed fromabove and has a slit; a display region in which the liquid crystal layeris driven by an electric field generated between the lower electrode andthe upper electrode; and a non-display region that is formed outside thedisplay region; wherein each of the common wirings is provided on a sideof, and spaced a distance apart from, one of the scanning lines, whereinthe lower electrode covers the common wirings in a manner that aplurality of steps are formed on at least one lateral surface of thecommon wiring, and wherein a dummy pixel is formed in the non-displayregion, and the step part of the lower electrode is formed in the dummypixel.
 2. The liquid crystal display device according to claim 1,wherein a width of a step part of the lower electrode is 2.25 μm or morefrom the lateral surface of the common wirings.
 3. The liquid crystaldisplay device according to claim 1, wherein the step part of the lowerelectrode is formed only near a position on which the common wirings andthe signal lines intersect with each other.
 4. The liquid crystaldisplay device according to claim 1, wherein the first insulation filmis formed such that when a thickness of an insulation film formed on aflat part on the lower electrode is denoted as A and a thickness of aninsulation film formed on a lateral surface of the steps of the lowerelectrode is denoted as B, a step coverage of the first insulation filmsatisfies a relationship of B/A≧1.